Teach 600 people to eat more vegetables and less added sugar for a year, and it turns out they’ll lose weight. There’s nothing super shocking in the DIETFITS study published today, but there’s a lot to learn about what doesn’t matter in dieting.

Both groups ended up eating fewer calories without being told to count or reduce calories.

People’s insulin response (how much insulin their body releases in response to a dose of sugar) did not predict which diet they would do better on.

Three DNA variations that were supposed to predict your success...also meant nothing.

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The study itself is paywalled, but Examine.com has a free breakdown of the study’s design and results. It’s the kind of study that other diet studies wish they were. It’s pretty big—300 people per group. It’s a randomized controlled trial, which means people were told which diet to do. The investigators used more reliable methods than just asking people what they ate. They also focused on getting solid answers to a few important questions they identified ahead of time—yielding the results in the bullet points above—rather than asking a million questions and fishing for blips in the data.

The Diets That Worked

The low-fat dieters were told to reduce their fat intake to 20 grams per day. That’s what’s in four teaspoons of olive oil, or three-quarters of an avocado. A single McDonald’s double cheeseburger would put you way over the limit.

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The low-carb dieters were told to reduce their carbs to 20 grams, as well. That puts you into ketogenic territory, where your body alters its metabolism to supposedly become a fat burning machine. For comparison, half a cup of rice has 22 grams of carbs.

Both groups went to classes taught by registered dietitians, one hour a week at first and less often as the year progressed. In the first eight weeks, they were taught to get their intake of fat or carbs (as appropriate) as low as possible. After that, they were to slightly increase the fat or the carbs to find out how much they could tolerate while still losing weight and enjoying their food. Here’s what they learned in the classes, according to a description of the study published last year:

Those assigned to Healthy Low-Fat were instructed to choose whole-grain foods (e.g., rather than whole wheat flour products), including steel cut oats, farro, barley, quinoa, brown rice, and wild rice. Healthy Low-Fat participants were also encouraged to explore and consume a wide range of legumes and beans, fresh fruit, low-fat dairy products, and lean meats. Those assigned to Healthy Low-Carb were instructed to choose high quality oils and fats, avocados, hard cheeses, nut butters, and nuts & seeds. During the Titrate phase [after the initial 8 weeks], and throughout the remainder of the 12-month protocol, as the Healthy Low-Fat group added small amounts of fat back to the diet, and as the Healthy Low-Carb group added small amounts of carbohydrate back to the diet, they were instructed to do so with these same quality foods.

Given that high quality foods can be more expensive than foods that are similar in type but lower in quality, the encouragement to choose quality was framed as a continuum as opposed to an either/or (e.g., for the Healthy Low-Fat participants, organic wheat berries was at the highest level of quality, followed by conventional wheat berries, then whole wheat bread made with a minimal number of ingredients and no additives, then a more conventional whole wheat bread with many ingredients including additives, and, finally, refined white flour bread with many ingredients and additives was considered the lowest end of the quality continuum). In other words, participants were encouraged to choose the highest quality foods that they could reasonably find, realistically afford, and enjoy.

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Most people didn’t actually achieve the 20 gram goal. The low-carb dieters averaged 96.6 grams of carbs at the end of the eight weeks, and increased that to 132 grams by the end of the year. The low fat dieters managed 42 grams of fat at the end of the first stage, and were eating 57 grams per day by the end of the year.

But even though they didn’t quite hit their targets, most people lost weight, whatever group they were in. The averages for both groups were around 12 to 13 lost pounds for the year, but there was a lot of individual variation! The examine.com folks put this graph together showing the results. In each group, a few people lost more than 55 pounds, and a good chunk of the participants didn’t lose weight at all, or even gained up to 22 pounds.

Numbers are in kilograms. A 25 kilogram loss is 55 pounds.

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What Doesn’t Matter

This study doesn’t bring us any closer to knowing why some people respond better than others to the diet. And since each person only tried one of the diets, we don’t know whether the people who gained weight on one diet would have done better on the other.

The researchers were hoping that genetics would reveal the answer. Three genes, PPARG, ADRB2, and FABP2, were identified in previous studies as being possibly related to whether people would do better on one diet or the other. So they looked at the SNPs, or gene variants, known as rs1801282, rs1042714, and rs1799883. If a person had the versions of all three that have previously been associated with low fat diet success, and they ended up on the low fat diet, they were considered to have genes that “matched” that diet. Same with the low carbers. Anybody whose genes were a mix of low-fat and low-carb favoring genes was considered mismatched to their diet.

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Sadly, this didn’t work out at all. The lead researcher told STAT: “We were so excited and thought this would work,” but after viewing the results, had to admit: “We didn’t even come close.”

(Can I just say, from the bottom of my science nerd heart, that it is excellent that a study with negative results is still being published and talked about, and the researchers aren’t trying to spin it into something it’s not? It’s super useful to know what doesn’t work, especially in the era of dubious genetics-based diet coaching.)

Insulin secretion also didn’t predict success on one diet or the other. When you eat food, and especially carbs, your pancreas squirts insulin into your blood as a signal for other cells to absorb the rush of nutrients. People with diabetes either don’t make enough insulin (type 1), or their fat and muscle cells don’t respond to the insulin (type 2).

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So they tested what happens to a person’s insulin levels 30 minutes after they drank a massive dose of sugar. Since type 2 diabetes is associated with obesity, the researchers wanted to test the theory that people who overproduce insulin will do better on a low-carb diet that doesn’t trigger as much insulin production. But this didn’t pan out either.

What does this mean for me?

First, this study was done in a specific population of people, so we don’t know if the results apply to everyone. Participants lived in the Stanford and San Francisco Bay areas of California. They tended to be fairly well educated and typically had enough money to easily afford healthy food.

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The study only included people with a BMI between 28 and 40, who were between the ages of 18 and 50, and they did not accept people who had diabetes, heart disease, cancer, uncontrolled high blood pressure, or who were pregnant or lactating. That leaves off a lot of people who want to lose weight, and in fact, a lot of reasons why people want to lose weight (to control their heart disease, or their diabetes, or to lose the baby weight.)

Both diets involved a focus on eating lots of veggies and less sugar, but this study doesn’t prove whether that’s enough; it might be that you also need a nutrient to minimize. I would have loved to see a control group that wasn’t given a goal to lower carbs or fat, but just given the baseline advice about a high quality diet.

And ultimately, we don’t really know why some people lost 55 pounds and others gained more than 20. The researchers are planning more analyses to look for more answers. So far, though, our best guess is that if you flip a coin your chances are probably just as good on a low-fat diet as a low-carb one.

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Updated 7am 2/21/2018 to correct the name of the journal and the units on some of the weight loss numbers (people lost up to 25 kilograms, not pounds—big difference!)